1. Field of the Invention
This invention relates to the technical field of transistor circuits including a plurality of transistors such as thin-film transistors (mentioned as TFT hereafter), field effect transistors and bipolar transistors, and particularly relates to the technical field of transistor circuits including driving transistors for controlling driving current, by controlling conductance between the source and the drain in response to voltage supplied to the gate, that is supplied to a driven element such as a current-controlled (current-driven) element through the source and the drain.
2. Description of Related Art
Generally, the voltage/current characteristics and thresholds of transistors tend to vary, depending on various conditions such as the quality and thickness of semiconductor films, impurity concentration and diffusion areas, the quality, thickness and the like of gate insulating films, operating temperature, and the like. In the case of bipolar transistors consisting of crystal silicon, the variance of thresholds is relatively small, but in the case of TFTs, the variance is usually large. Particularly, in the case of TFTs formed in a wide range in plurality on a TFT array substrate in a display panel such as a liquid crystal panel, an EL panel, and the like, the variance in voltage/current characteristics and thresholds often becomes extremely large. For instance, when such TFTs are manufactured so as to set the threshold at about 2V (+2V in the case of N channel, and xe2x88x922V in the case of P channel), the variance is sometimes about several xc2x1V.
In the case of a voltage-controlled (voltage-driven) type transistor for controlling the voltage of picture elements made of liquid crystals or the like, such as a so-called TFT liquid crystal panel, the variance in voltage/current characteristics and thresholds of driving TFTs that are applied to each picture element is not likely to be a problem. In other words, in this case, even if there is a slight variance in the voltage/current characteristics and thresholds of TFTs, contrast and brightness of each picture element can be controlled at high precision by increasing the precision of the voltage supplied to each picture element from the outside through the TFTs only if there is enough switching time. Therefore, even in the case of a TFT liquid crystal panel or the like for display wherein contrast and brightness at each picture element are regarded as important, high grade picture images or the like can be displayed by TFTs with a relatively large variance of voltage/current characteristics and thresholds.
On the other hand, display panels have been recently developed that include current-controlled light-emitting elements, such as a self light-emitting organic ELs to change the brightness at picture elements in response to current supply. These display panels have received attention as display panels that can display picture images without back light and reflected light, that consume less power, being less dependent on the angle of view, and are sometimes flexible. Even in this EL panel, a driving TFT is used at each picture element for driving an active matrix. For instance, it is constructed so as to control (change) the driving current supplied to an EL element from power source wiring connected to a source in response to the voltage of data signals applied to a gate, by connecting the drain of a driving TFT to the EL element through a hole-injecting electrode. Using a driving TFT as mentioned above, driving current flowing to an EL element can be controlled by controlling conductance between a source and a drain in response to the voltage change of input signals, so that brightness at each picture element can be changed for picture image display and the like.
However, particularly in the case of the current-controlled element such as the EL panel mentioned above, the variance of voltage/current characteristics and thresholds tends to be a problem in the driving TFT at each picture element. In other words, in this case, even if the voltage precision of data signals supplied to the driving TFTs from the outside is enhanced to some extent, the variance in voltage/current characteristics and thresholds in the driving TFTs appears directly as the variance of the driving current supplied to data signals, thus reducing the precision of the driving current. As a result, the brightness at each picture element is likely to vary in accordance with the variance in thresholds of the driving TFTs. Moreover, especially with current manufacturing techniques of low temperature polysilicon TFTs, voltage/current characteristics and thresholds vary considerably. Thus, this problem is, in practicality, extremely serious.
If each TFT is manufactured so as to reduce the variance in voltage/current characteristics and thresholds in consideration of this problem, the yield will decline and, particularly in the case of an apparatus with a display panel having a plurality of TFTs, the yield will decrease a great extent, and thus opposing a general goal of lower costs. Alternatively, it is almost impossible to manufacture TFTs that can lower such a variance. Moreover, even if a circuit for compensating the variance of voltage/current characteristics and thresholds at each TFT is installed separately, the apparatus will be complex and large, and moreover, the consumption of electric power will increase. Particularly, in the case of a display panel wherein a plurality of TFTs are arranged at high density, the yield will decline again or it will be difficult to satisfy current demands such as lower power consumption, and miniaturization and lightening of an apparatus.
This invention is carried out in consideration of the above-noted problems, and aims to provide transistor circuits for controlling conductance in driving transistors in response to the voltage of input signals, the conductance of which can be controlled by relatively small input signals and that can compensate for the variance in voltage/current characteristics and thresholds of driving transistors with somewhat smaller power consumption by using a relatively small number of transistors, and a display panel and an electronic apparatus using the same.
In this invention, the following transistor circuits according to the first to tenth aspects are provided.
First, according to a first aspect of the invention a transistor circuit is characterized in that it includes a driving transistor having a first gate, a first source and a first drain, wherein conductance between the first source and first drain is controlled in response to the voltage of input signals supplied to the first gate, and a compensating transistor having a second gate, a second source and a second drain, wherein the second gate is connected to one of the second source and second drain, and wherein the compensating transistor is connected to the first gate in an orientation so as to supply the input signals to the first gate through the second source and second drain, and to allow the first gate to move electric charge into a direction to lower the conductance.
According to the above-noted transistor circuit of the first aspect of the invention, one of the second source and second drain of the compensating transistor is connected to the first gate of the driving transistor, and input signals are supplied to the first gate of the driving transistor through this second source and second drain. Then, at the driving transistor, the conductance between the first source and first drain is controlled in response to the voltage of input signals supplied to the first gate. Herein, the compensating transistor has the second gate connected to the second drain, and is connected to the first gate in an orientation to allow the first gate to move electric charge into a direction to lower the conductance between the first source and first drain. In other words, the compensating transistor has diode characteristics and when the driving transistor is, for example, an N-channel type transistor, current can be carried from the first gate into the direction of an input signal source. Alternatively, when the driving transistor is a P-channel type transistor, current can be carried from an input signal source to the direction of the first gate.
Therefore, as input signals are supplied to the transistor circuit, the gate voltage of the first gate, compared with the voltage of input signals at the time of being input to the compensating transistor, rises to the side of increasing the conductance of the driving transistor only by a threshold level of the compensating transistor. As a result, in order to obtain preferable conductance in the driving transistor, input signals of a voltage that is lower only by a threshold (voltage) level of the compensating transistor, instead of the gate voltage corresponding to the conductance, can be supplied through the compensating transistor. In this way, since the gate voltage in response to input signals can rise only by a threshold (voltage) of the compensating transistor, equivalent conductance control can be carried out by the lower voltage of input signals compared with the case of no compensating transistor.
These input signals are generally at a high frequency relative to other signals, and the consumption of electric power can be reduced significantly if lower input signals can be used.
Moreover, setting a gate voltage at the first gate by increasing the voltage of input signals from the compensating transistor as mentioned above indicates that, when seen as a transistor circuit as a whole, the threshold of input signals supplied to a driving current flowing through a source and a drain whose conductance is controlled in the driving transistor is lower than the threshold voltage of the driving transistor only by the threshold voltage of the compensating transistor as a voltage increases from the input voltage to the gate voltage. In other words, within the threshold of input voltage supplied to a driving current, the threshold of the compensating transistor and the threshold of the driving transistor are offset from each other. Therefore, by making the threshold characteristics and the voltage/current characteristics of both transistors similar to each other, it is possible to set the threshold of input signals to driving current to zero.
Moreover, by offsetting the threshold of the driving transistor and the threshold of the compensating transistor in the transistor circuit as a whole as mentioned above, the threshold of input signals can be set closer to a constant level (zero) without depending on the level of threshold of the driving transistor. In other words, when a plurality of transistor circuits is prepared by using many driving transistors with different thresholds, a difference in thresholds between transistor circuits is smaller than (or is ideally almost the same as) a difference in the thresholds of driving transistors by setting the thresholds of the driving transistor and the compensating transistor in each transistor circuit close to each other (ideally equal to each other). Thus, in preparing a plurality of transistor circuits, a plurality of transistor circuits with almost or completely no variance in thresholds can be provided even when many driving transistors with many different thresholds are used.
According to a second aspect of the invention, the transistor circuit according to the first aspect mentioned above is characterized in that it has a resetting means for supplying reset signals, having a voltage that gives higher conductance than the maximum conductance controlled in response to the input signals, to a first gate before the input signals are supplied.
According to the above-noted transistor circuit of the second aspect of the invention, before input signals are supplied to the first gate of a driving transistor (or after the input signals are supplied before the next input signals are supplied), reset signals, having a voltage which gives a higher conductance than the maximum conductance of the driving transistor controlled in response to input signals, are supplied to this first gate by the resetting means. As a result, the gate voltage of the driving transistor can be set constant without depending on the level of voltage of input signals. Moreover, it becomes possible to supply input signals to the first gate through the compensating transistor which is connected to the first gate in an orientation to permit electric charge to move into a direction so as to lower conductance after resetting.
According to a third aspect of the invention, the above-noted transistor circuits according to any of the first and second aspects is characterized in that the reset signals are set at a voltage higher than the maximum voltage of input signals by a threshold voltage level of the compensating transistor.
According to the above-mentioned transistor circuit of the third aspect of the invention, reset signals having a higher voltage than input signals are supplied to the first gate of the driving transistor by the resetting means. Moreover, the voltage of these reset signals is set higher than the maximum voltage of the input signals by a threshold voltage of the compensating transistor, so that a voltage higher than the voltage of the input signals by a threshold voltage level of the driving transistor can always be supplied to the first gate of the driving transistor through the compensating transistor, without being dependent on the level of voltage of input signals or thresholds of the driving transistor, when input signals are input after resetting.
According to a fourth aspect of the invention, the transistor circuit according to the second aspect mentioned above is characterized in that it includes a resetting transistor wherein the resetting means has a third gate, a third source and a third drain, wherein one of the third source and the third drain is connected to the first gate, and wherein the reset signals are supplied to the first gate through the third source and third drain after reset timing signals are supplied to the third gate before the supply of the input signals.
According to the above-noted transistor circuit of the fourth aspect of the invention, when reset timing signals are supplied to the third gate of the resetting transistor, reset signals are supplied to the first gate of the driving transistor through the third source and third drain by the resetting transistor. As a result, the gate voltage of the driving transistor can be reset at a constant by the timing of supplying reset timing signals. Therefore, the operations that are explained for the second transistor circuit become possible.
According to a fifth aspect of the invention, the transistor circuit according to any one of the first to the fourth aspects mentioned above is characterized in that the driving transistor and the compensating transistor are the same type of transistors.
According to the transistor circuit of the fifth aspect of the invention mentioned above, the driving transistor and the compensating transistor are the same type of transistors, but the xe2x80x9csame typexe2x80x9d means that the conductive type of transistors is the same herein. For instance, when the driving transistor is an N channel type transistor, the compensating transistor is also an N channel type transistor. With the deriving transistor as a P channel type transistor, the compensating transistor is also a P channel type transistor. Therefore, the threshold of the compensating transistor and the threshold of the driving transistor become almost equal to each other, so that these thresholds are offset from each other in the transistor circuit. As a result, it becomes possible to carry out conductance control by setting the threshold of input signals supplied to a driving current to approximately zero.
Also, by providing the same transistor channel width, design values including a channel length, device structures, process conditions, and the like to both the driving transistor and the compensating transistor, more complete compensation becomes possible.
According to the sixth aspect of the invention, the transistor circuit according to any of the above-noted first to the fifth aspects is characterized in that the circuit further includes a switching transistor having a fourth gate, a fourth source and a fourth drain, and wherein the transistor is connected so as to supply the input signals to the compensating transistor through the fourth source and fourth drain when switching timing signals are supplied to the fourth gate.
According to the above-noted transistor circuit of the sixth aspect, when switching timing signals are supplied to the fourth gate of the switching transistor, input signals are supplied to the compensating transistor through the fourth source and fourth drain of the switching transistor. As a result, input signals can be supplied to the driving transistor by the supply timing of switching timing signals.
According to a seventh aspect of the invention, the transistor circuit according to any of the above-noted first to sixth aspects is characterized in that it further includes a storage capacitor connected to the first gate.
According to the transistor circuit of the seventh aspect, when input signals are supplied to the first gate, the voltage is held by the storage capacitor connected to the first gate. Therefore, even when input signals are supplied only for a fixed period, the voltage at the first gate can be held over a longer period than the fixed period.
Also, even when there is leakage current in the switching transistor through the compensating transistor, it becomes possible to reduce the fluctuation of electric potential applied to the first gate.
According to an eighth aspect of the invention, the transistor circuit according to any of the above-noted first to seventh aspects is characterized in that the transistors consist of thin-film transistors formed on the same substrate respectively.
According to the transistor circuit of the eighth aspect, the effect of voltage/current characteristics and threshold characteristics of the driving thin-film transistor, which is formed on the same substrate, on a driving current can be compensated by the compensating thin-film transistor. Particularly, as both thin-film transistors are formed on the same substrate in the same thin-film forming process, the characteristics between both transistors become similar, so that it becomes possible to provide a plurality of transistor circuits with little variance in voltage/current characteristics and threshold characteristics on the same substrate.
According to a ninth aspect of the invention, the transistor circuit according to any of the above-noted first to seventh aspect is characterized in that the transistors consist of bipolar transistors respectively, wherein the gate, source and drain correspond to a base, a collector and an emitter respectively.
According to the transistor circuit of the ninth aspect, the effect of voltage/current characteristics and threshold characteristics of the driving bipolar transistor on a driving current can be compensated by the compensating bipolar transistor. Particularly, as both bipolar transistors are manufactured in the same manufacturing process, the degree of characteristic similarity between both transistors generally increases, so that it becomes possible to provide a plurality of transistor circuits with little variance in voltage/current characteristics and threshold characteristics.
According to a tenth aspect of the invention, the transistor circuit according to any of the above-noted first to ninth aspects is characterized in that the input signals are voltage signals where the voltage is controlled by an input signal source and that the driving transistor, wherein one of the first source and first drain is connected to a current-controlled element, and electric current flowing to the current-controlled element is controlled by controlling the conductance.
According to the transistor circuit of the tenth aspect of the invention, as the voltage signals where a voltage is controlled by an input signal source are supplied through the compensating transistor as input signals, conductance between the first source and first drain is controlled in response to the change in voltage of these voltage signals in the driving transistor. As a result, the current-controlled element connected to one of the first source and first drain is current-controlled. Thus, it becomes possible to current-drive the current-controlled element by the input signals of a relatively low voltage. Moreover, it becomes possible to current-control a plurality of current-driven elements with good precision in response to voltage signals without being dependent on the variance in voltage/current characteristics and thresholds between a plurality of driving transistors.
According to this invention, a display panel is provided which is characterized in that it includes the above-noted tenth transistor circuit of this invention respectively and has a plurality of picture elements arranged in a matrix, and that current-controlled light-emitting elements are provided respectively to the plurality of picture elements as the current-controlled elements.
According to the display panel, as input signals are provided through the compensating transistor at each picture element, the current-controlled light-emitting elements are current-controlled in response to the voltage of these input signals by the driving transistor, so that the brightness of the current-controlled light-emitting elements can be controlled with good precision without being dependent on the variance in voltage/current characteristics and threshold characteristics among the driving transistors, and that the unevenness of brightness can be reduced over the entire screen display area of the display panel. Moreover, by increasing the gate voltage of the driving transistor with the compensating transistor, the current-controlled light-emitting elements can be controlled by the input signals of a relatively low voltage.
According to this invention, an electronic apparatus having the above-noted display panel is provided.
According to such an electronic apparatus, since it has the above-described display panel, an electronic device can be realized that has little unevenness in brightness over the entire surface of the display panel and can be driven at a relatively low voltage.